It is currently known to prepare methyl methacrylate from isobutyric acid by a two-stage process.
In the first stage, the oxidative dehydrogenation of isobutyric acid is carried out in the vapor phase at a temperature between 300.degree. C. and 450.degree. C. over a catalyst which may be either derived from iron phosphates as described, for example, in U.S. Pat. No. 4,298,755, or may be derived from phosphomolybdic acid as described for example in European Pat. No. 0,194,541.
The conversion of isobutyric acid is generally from 85% to 98%. Diluting water is generally necessary to keep the catalyst active, and water is formed during the reaction. The product leaving the oxidative dehydrogenation reactor, after condensation, consists chiefly of methacrylic acid, of unconverted isobutyric acid, of a greater or lesser proportion of water (25% to 85%), together with acetone and acetic acid in smaller proportions. A known manner of separating water from the organic acids consists in performing a liquid-liquid extraction. Many extraction solvents are described in the literature for this purpose: esters, ketones, aromatic solvents and mixtures thereof may be mentioned in particular.
However, a certain number of separations must still be carried out to obtain a methacrylic acid intended to be subjected to the second stage; thus, acetone must be removed and it is desirable to remove acetic acid and isobutyric acid as well.
In the second stage, the esterification of methacrylic acid with methanol to methyl methacrylate is carried out by an equilibrium reaction in liquid phase at a temperature from 70.degree. C. to 130.degree. C. in the presence of an acidic catalyst which may consist, for example, of sulfuric acid, of paratoluenesulfonic acid, of methanesulfonic acid or, preferably, of strong cationic resins. At the reactor outlet, methyl methacrylate, methacrylic acid, water and methanol are recovered possibly together with acetic and isobutyric acids and esters, depending on the degree of purification of the methacrylic acid introduced. A column is generally placed at the reactor outlet, enabling the unconverted acid or acids to be taken off so that they can be recycled to the esterification reactor.
Purification of methyl methacrylate is then performed according to known processes such as distillations, possibly a liquid-liquid extraction, and the like.
The process for the manufacture of methyl methacrylate in the form just described has the major disadvantage that isobutyric acid is incompletely converted into methacrylic acid in the first stage, with the result that the final methyl methacrylate contains methyl isobutyrate as an impurity in a quantity which is higher than the permitted limit for a methyl methacrylate to be considered as a high-purity commercial product. Such a product must, in fact, contain fewer than 100 parts per million of nonpolymerizable substances; it must, in other words, have a methyl isobutyrate content lower than this threshold of 100 parts per million.
Two possibilities offer themselves in order to obtain this very low content of methyl isobutyrate.
It is possible to consider separating methacrylic acid from isobutyric acid at the end of the first stage until a methacrylic acid of sufficient purity, that is to say containing fewer than 100 parts per million of isobutyric acid, is obtained. It is also possible for the isobutyric acid obtained at the head of the column to be then recycled to the oxidative dehydrogenation reactor to be combined with the initial isobutyric acid.
However, the technology of this distillation is difficult to implement. Measurements of liquid-vapor equilibrium show that, as a result of a nonideal thermodynamic behavior, the separation requires a very large number of theoretical plates. At the same time, because of the sensitivity of methacrylic acid to polymerization and of its high boiling point, this distillation must be conducted under high vacuum and using equipment which performs the separation with a very low pressure drop.
It is also possible to consider not modifying the first stage, in which case a mixture of methacrylic acid and unconverted isobutyric acid are introduced to carry out the esterification in the second stage. These acids are converted into esters in an equivalent proportion and methyl isobutyrate must then be separated form methyl methacrylate until the desired purity is obtained.
This separation offers a lesser technological difficulty than the separation of isobutyric acid, referred to above. On the other hand, on an industrial scale it is of no advantage to dispose of the methyl isobutyrate which would be recovered at the head of the column. Burning it leads to an unacceptable economic waste and, if it is recycled to the oxidative dehydrogenation reactor, the unconverted isobutyrate and the methyl methacrylate obtained will create a major disturbance in the first stage.
Therefore, neither of the above two possibilities in order to improve the known process with a view of obtaining methyl methacrylate of high purity appears to be satisfactory. Research has therefore been conducted for an acceptable solution to the problem which was presented and this has led to the present invention.